Smells and aromas are profoundly evocative. Those related to Christmas are often connected to our childhood, able to bring us back (at least with the memory) to that awesome time of our lives. This year, we need more than ever this powerful tool to escape at least for a little bit the harsh present reality.
What gives Christmas its special smell and aroma? Besides the scent coming from Christmas trees, made up by a blend of mono and sesquiterpenes, we must doubtless mention the spices used for preparing sweets and drinks. Each Country has its own traditions, often even different among regions and areas. However, there are some spices that are frequently used, alone or mixed together, to give that specific Christmassy aroma, like cinnamon, clove, ginger, nutmeg, and star anise. The aroma of each of these spices is composed by a mix of volatile compounds, but all of them also have one specific and characteristic main chemical.
Cinnamaldehyde is a phenylpropanoid that gives its characteristic aroma to a spice present in almost any Christmas cookie and sweet, cinnamon, obtained from the bark of some species of Cinnamomum (Lauraceae) trees. Many biological activities are attributed to cinnamaldehyde. It has antifungal, antibacterial and antibiofilm properties (1,2), insecticidal and insect repellent activity (3). It has also anti-inflammatory, hypoglycaemic and hypolipidemic properties (4,5), although this has only been demonstrated in animal models. The role of cinnamaldehyde in the tree is not clear yet, although it might be related to some of the above-mentioned activities, with a defensive action against pathogens and predators.
Eugenol is the main responsible of the characteristic smell of the dried flower bud of the clove tree, Eugenia caryophyllata (now Syzygium aromaticum, Myrtaceae). Analogously to cinnamaldehyde, it is a phenylpropanoid with several biological activities (6). It is used as a flavouring agent and as a component of cosmetics and soaps. Furthermore, it is used as a local antiseptic and anaesthetic. Eugenol also finds applications in agricultural practices as a pesticide and fumigant and it can be used as a food additive to protect it from microorganisms during storage (6). Besides its possible role as plant defensive chemical, eugenol has also an interesting function in bees, particularly in a species of South American orchid bees. These insects are thought to collect eugenol and other plant volatiles and to use them as pheromones, chemicals used to trigger a social response in other members of the same species (7). Alternatively, they can use the plant chemicals to synthesize these pheromones, with small chemical modifications.
The spiciness of ginger (Zingiber officinale), is due to gingerol, which has a mechanism of action analogous to that of capsaicin. Several gingerols of various chain lengths (n6 to n10) are present in ginger, with the most abundant being 6-gingerol (8). The dried ginger contains a few derivatives of gingerol that make it even more spicy. During cooking, gingerol undergoes a reverse aldol reaction, giving zingeron, which is responsible for the typical sweet spicy aroma of gingerbread. Gingerols and their derivatives seem to have antioxidant, anti-inflammatory, but also antiobesity and antidiabetic activities and protective effects against respiratory disorders (8). However, many of these properties need further evidence. Gingerols also have antimicrobial activity, which could explain, along with the antioxidant properties, the use of ginger essential oils as food preservatives (9). Concerning the role in planta, although it is not clear yet, a protective role (especially as feeding deterrent) is plausible, but studies are needed to prove this.
Nutmeg is a spice native to the Moluccas Islands (Indonesia). Its distinctive nutty, slightly sweet, warm aroma is due especially to myristicin, which has a mild narcotic and hallucinogenic effect. Once ingested, it is thought to be metabolized into a compound structurally related to amphetamine (and in particular to ecstasy), 3-methoxy-4,5-methylenedioxyamphetamine (MMDA) (10), although this has only been demonstrated in rat livers. Due to the hallucinogenic effect of nutmeg, there is an history of its abuse that often resulted in intoxications. However, the doses used for cooking are safe. Myristicin is also an insect repellent and it has insecticidal activity (11). Activity that it could also have in planta, although this has not been extensively explored yet.
Anethole is the main compound responsible for the aroma of star anise. Star anise (Illicium verum) is used not only as a spice, but also as a decoration, due to its peculiar and beautiful shape. It is a plant native to China and Vietnam, where it is largely used in the cuisine and in traditional medicine. Anethole is broadly used in cosmetics and soaps, but recently also several roles in human health have been proposed and are under investigation (12). It has antifungal activity (13) and although its original function is unknown, it might be involved in the plant protection against pathogens.
Cinnamaldehyde, eugenol, gingerol, myristicin, and anethole are all phenylpropanoids responsible for the aroma of many sweets and drinks that contribute to the warm atmosphere of Christmas time, although they are originally made by the plants for very different reasons (sometimes still unknown). Let’s all enjoy some nice food and drinks containing these compounds and, at least for a while, create a cheerful environment and forget about our sorrows…but, hey, nutmeg only allowed for cooking!